1. Field of Invention
The present invention relates to a method for bonding metal to ceramics and, more particularly, to a method for bonding stainless steel to aluminum oxide.
2. Related Prior Art
In general, metal exhibits reliable mechanical strength, water-tightness, thermal conductivity and electric conductivity. Metal may be bonded to metal, ceramics or glass to improve thermal performance, electric performance, mechanical strength and/or air-tightness, and/or reduce thermal expansion.
Ceramics is refractory, chemically stable, anti-oxidation, electrically isolative, dense and optical. Ceramics may be made from different components in different processes and used in the electronic industry, the aerospace industry, the nuclear industry, the automobile industry, fuel cells, cutting tools, or the biotechnology.
In many cases, metal is bonded to ceramics. In these cases, wearing, erosion, scratching, oxidation, thermal resistance, air-tightness, mechatronics and reliability are be taken into consideration. Bonding of metal to ceramics is often used in the defense industry, automobile electronic parts, sealing of photoelectric elements or semiconductor, the aerospace industry, fuel cells, cutting tools, the chemical industry, the environmental protection, optical fibers for telecommunication, and the biotechnology.
There are however problems related to residual thermal stress and wetability. For example, the thermal expansion coefficient of AISI 316 stainless steel is 18×10−6K−1 while the thermal expansion coefficient of aluminum oxide (“Al2O3”) is 6×10−6K−1. There is a big difference between the thermal expansion coefficient of aluminum oxide and that of AISI 316 stainless steel. Brazing for bonding aluminum to AISI 316 stainless steel is a big challenge in the material science and engineering.
Ag—Cu—Ti alloy is often used as active brazing solder for bonding aluminum oxide to AISI 316 stainless steel in the semiconductor industry that requires ultra vacuum tightness. Such bonding can only be executed in small areas or in a slowly heating or cooling process, and the Al2O3 must be subjected to sintering and hot isostatic pressing for excellent mechanical strength. The Al2O3 requires mechanical strength against thermal stress so that the thermal stress, although it may be high, does not exceed the mechanical strength of the Al2O3 to ensure the success of the bonding. If the quality of the Al2O3 is poor, i.e., the mechanical strength of the Al2O3 varies from region to region, cracks will almost certainly occur in weak points of the Al2O3 and finally cause the bonding to fail. Obviously, to successfully bond together elements of different values of mechanical strength against thermal stress, the mechanical strength of the elements must be high or there must be an analyzed, solid-solution or diffused solder seam.
Recently, ceramics develops fast and is used in various fields, and many processes have been used to bond metal to ceramics such as diffusion bonding, brazing and welding. Among these processes, brazing is deemed the best process to bond metal to ceramics. Base materials are not molten in brazing so that brazing can be used to bond together materials that cannot be bonded together by welding.
To bond metal to ceramics by brazing, there is however a problem that most sorts of brazing solder cannot wet the ceramics effectively and the interface between the brazing solder and the ceramics is not strong enough. To improve the wetabilty, there are several approaches. Mo—Mn metallization process has been applied with long history for ceramic-metal joining in industry. The Mo—Mn process involves the metallization of ceramic materials by the mixture of Mo and Mn or MnO2 at higher temperature of 1450° C. under controlled humidity, followed by brazing the metalized ceramic with steel. The ceramics may be metalized before the brazing. Alternatively, the brazing solder may be dosed with an active element such as titanium, zirconium and chromium that reacts with the ceramics in the brazing so that the brazing solder wets the ceramics. This process is called the “active brazing.”
There have been many researches for the active brazing for two reasons. At first, most sorts of ceramics are chemically stable, and traditional sorts of brazing solder fail to wet the ceramics adequately even though the surface of the ceramics is very clean. Secondly, the active element added in the brazing solder improves the wetability considerably.
The most popular active brazing solder is silver-copper eutectic crystal dosed with 2 wt % to 5 wt % of titanium. The silver-based active brazing solder wets most sorts of ceramics and forms good bonding. Because of the active element added to the brazing solder, reaction occurs between the ceramics and the brazing solder. The morphology, composition and thickness of the reactant influence the mechanical strength of the bonding. Hence, various mechanisms of fissures in bonding have been discussed in many papers based on theories and experiments. It has been found in the experiments that most fissures occur at the interface or in the ceramics.
In addition to the Mo—Mn process, the metal-ceramic joining has been executed by the active brazing method with the Ag-base active braze solder. For example, as disclosed by L. X. Zhang et al. in 2008, glass has been bonded to 30Cr3 stainless steel successfully by Ag-21 Cu-4.5Ti at 840° C. to 1000° C. for 5 minutes. The titanium reacts with the silicon, the oxygen and the iron to form Ti4O7, TiSi2 and TiFe2 to improve the bonding while the residual silver and copper are turned into solid solution or analyzed in the region of brazing. As the temperature rises, the thickness of the reaction layer increases.
For example, as disclosed by O. C. Paiva et al. in 2008, AISI 316 stainless steel has been bonded to 99.6% aluminum oxide by Ag-26.5Cu-3Ti (“CB4”) and Ag-34.5Cu-1.5Ti (“CB5”) successfully. The temperature may be 850° C., 900° C. or 950° C. The brazing lasts for 20 minutes. The rates for the temperature to rise and fall are 5° C. and 1.2° C., respectively. The best mechanical strength against shear is 234±18 MPa where CB4 is used and the brazing is executed at 850° C. For CB5, the best mechanical strength against shear is 224 MPa where the brazing is executed at 900° C. As the temperature rises, the thickness of the reaction layer and the concentration of the Ti at the interface fall. The reduction of the thickness of the reaction layer probably may be attributed to inadequate mechanical strength. Moreover, the heating and cooling rates still have to be low.
In researches for brazing, it has been found that in an early stage, at the interface, Ti reacts with aluminum oxide to form Ti3(CuAl)3O, not TiO2, and this reaction layer dissolves a large amount of Al. It is however not possible to precisely obtain the relation of the forming of this reaction layer to the dynamic wetting angle from the experiments. Influences on the mechanical strength of the bonding by the form of the interface require further exploring.
In a few papers, copper-based or silver-based brazing solder is dosed with an active element such as Zr and Hf to provide active brazing solder such as Cu-22% Ti alloy, Cu-10% Zr alloy, Cu-15% Hf alloy and Ag—Cu—Zr alloy. These sorts of active brazing solder may be used for brazing AlN, Mullite or ZrO2. There have however been very few analyses of the dynamic wetting and reaction at the interface between the active brazing solder and the ceramics, and further exploring is needed.
Moreover, Shirzada et al. has used stainless steel foam in brazing for bonding aluminum oxide to AISI 316 stainless steel. The mechanical strength against shear is only 33 MPa. As reported by Zhang et al. and Rohde et al., results of four-point bending tests on aluminum oxide bonded to stainless steel are 210 MPa and 80 MPa. As disclosed by Paiva and Barbosa, the maximum mechanical strength against shear is 234±18 MPa; however, there is a crack at the 316 SS/solder interface. In all of the papers on brazing for bonding stainless steel to aluminum oxide, in the brazing, the temperature is controlled to fall at a very low rate (<5° C./min). Kar et al. attributes the low mechanical strength of 64 to 94 MPa to the difficulty in controlling the reaction layer in the brazing. In a study by Do Nascimento et al. of Kovar/Al2O3 brazing where the difference between the thermal expansion coefficients is small, the mechanical strength in three-point bending tests is only 130 MPa.
As discussed above, two problems have been encountered in brazing for bonding metal to ceramics. At first, the ceramics is fragile and vulnerable to unexpected fissures. Secondly, As ceramics is fragile and metal is malleable, the joint of ceramic and metal exhibits poor mechanical strength against thermal stress. Hence, the ceramics and the metal must be carefully chosen so that the difference between the thermal expansion coefficients thereof is small to reduce thermal stress. Hence, it is desirable to bond Kovar or alloy with a low thermal expansion coefficient to Pyrex or ceramics. However, Kovar or the alloy with a low thermal expansion coefficient is vulnerable to erosion, poor mechanical strength at high temperature, and a high cost for including a lot of nickel. Theoretically speaking, it is very difficult to bond aluminum oxide to alloy of a high thermal expansion coefficient such as AISI 316 stainless steel.
To improve brazing for bonding metal to ceramics, attention is paid to mechanical bonding, chemical bonding or wetting the ceramics with brazing solder. Regarding the mechanical bonding, bosses are formed on the metal (or the ceramics) and recesses are defined in the ceramics (or the metal) so that the bosses can be fit in the recesses to enhance the bonding of the metal to the ceramics.
Regarding the chemical bonding, the metal is oxidized to form a layer of oxidation previously. The layer of oxidation reacts with the ceramics at high temperature to form chemical bonds.
About the use of the brazing solder, it is important to wet the ceramics. The wetability is determined by a contact angle. The wetability is good where the contact angle is acute (<90°). The wetability is poor where the contact angle is obtuse (>90°). However, most sorts of brazing solder do not wet ceramics effectively. As a result, the mechanical strength at the interface between the brazing solder and the ceramics is inadequate, and so is the bonding. Moreover, as the temperature rises, the thickness of the reaction layer and the concentration of Ti at the interface fall. Furthermore, in the brazing by providing the brazing solder between the ceramics and the metal, the temperature must be controlled to rise very slowly, and the dynamic wetting interface reaction is incomplete, and the control over the reaction layer is difficult.
Bonding of aluminum oxide to AISI 316 stainless steel has often been used in the industry. However, high temperature is required during the bonding, and this is a burden on the financial side. (ie:Mo—Mn metallization process has been applied.) To lower the temperature without jeopardizing the mechanical strength of the bonding, efforts are made in two aspects. At first, the wetability of the ceramics by the brazing solder should be improved. Poor wetability results in poor bonding. Secondly, the residual stress should be reduced. High residual stress results in high possibility of cracks or even breach in the ceramics after the bonding.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.